Funny how after a year in JMC i can't really answer most of those questions.
1. It should be possible : i know someone who transferred from computing to jmc, at christmas last year. ( to do so he had to pass the january maths exams). If you want to do it you should ask about the transfer quite soon.
The 30 max people thing shouldn't be an issue as most of the time someone doesnt' show up or switches to maths. We started at about 26 people this year.
2. The programming in JMC is the same as in Computing, in my opinion it is not too hard( at least first year) , i hadnt done any programming before and still managed to do quite well.
Sometimes we get a little more time for some assignments as JMC is pretty busy.
3. Dunno... almost everybody that manages to do the masters degree gets a 2.1 or higer, for the 3 year course i think it s about 50/50 ( this is based on this years results which i have seen online).
4. A few people everyyear i guess. This year a few people left during the year/ switched to computing only , and some others didn't pass their exams.
That s probably just the first year though , and i m not sure if it s the same everyyear.